CVD vacuum chambers are employed to deposit thin films on semiconductor substrates. A precursor gas is charged to a vacuum chamber through a gas manifold plate situate above the substrate, which substrate is heated to process temperatures, generally in the range of about 250.degree.-650.degree. C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon and to form volatile by-product gases, which are pumped away through the chamber exhaust system.
To increase manufacturing efficiency and device capabilities, the size of devices formed on a substrate has decreased, and the number of devices formed on a substrate has increased in recent years. Thus it is increasingly important that CVD deposited thin films be of uniform thickness across the substrate, so that all of the devices on the substrate are uniform. Further, it is increasingly important that the generation of particles in processing chambers be avoided to reduce contamination of substrates that will reduce the yield of good devices.
Further, the size of semiconductor substrates, e.g., silicon wafers, has increased so that the present state-of-the-art silicon wafers are about 8 inches in diameter. This makes it feasible to process only one wafer at a time in a processing chamber, as opposed to batch-type processes that process a plurality, up to 100 wafers, at a time. The economies of batch processing are reduced using large wafers because if there is a problem during processing, many expensive wafers are damaged and must be discarded. Further, the processing chambers can be made smaller when only one substrate is to be processed at a time, and the processing is more controllable.
Still further, equipment has been developed to automate wafer processing by performing several sequences of processing steps without removing the wafer from a vacuum environment, thereby reducing transfer times and contamination of wafers. Such a system has been disclosed for example by Maydan et al, U.S. Pat. No. 4,951,601, in which a plurality of processing chambers are connected to a transfer chamber. A robot in a central transfer chamber passes wafers through slit valves in the various connected processing chambers and retrieves them after processing in the chambers is complete.
A typical prior art CVD chamber is disclosed in FIG. 1. This chamber is described in U.S. Pat. No. 4,892,753 to Wang et al, incorporated herein by reference. Referring to FIG. 1, a susceptor 16 on which a wafer 14 is mounted during processing is movable in the chamber 10 by means of a vertically moveable elevator (not shown). A plurality of pins 20 support the wafer as the wafer is brought into the chamber from an external robot blade. A plurality of susceptor support fingers 22 are connected to the wafer fingers 20 and are mounted on a bar 40, which is also vertically moveable by the elevator. The wafer 14 and the susceptor 16 on which it is mounted are heated by a plurality of high intensity lamps 58 through a light transmissive quartz window 70. In a preferred configuration, two banks of lamps are located outside both the top and the bottom of the chamber 10, when there are two sets of quartz windows 70. These quartz windows 70 are sealed to the chamber walls by means of Teflon seals 72. The use of these external heating lamps 58 allows very rapid heating of wafers and susceptors, and allows the chamber to be cooled between processing cycles when the lamps are turned off.
However, the quartz windows 70 have a comparatively short lifetime; after about 1000-2000 deposition cycles, sufficient deposition occurs on the quartz windows so that they cloud over, and the light from the high intensity lamps can no longer penetrate the quartz windows 70, when they must be cleaned. The quartz windows are also attached by flourine-containing plasma used to clean the chamber, which also generates particles. A flow of purge gas across the windows 70 has extended the period between cleanings, but the downtime required for cleaning or replacement of the windows 70 is still expensive.
In addition, the high intensity lamps 58 must be periodically replaced as well, causing additional downtime of the equipment.
Another problem with the use of the prior art CVD chambers is that they take a long time, up to about 6 hours, to degas so as to maintain a low leakage rate in the chamber. The Teflon seals 72 used to seal the quartz windows 70 are helium permeable, and outgas slowly, and it takes a long period of time for the chamber 10 to reach a satisfactory vacuum integrity. The present standards for chamber leakage require that the pressure inside the chamber 10 be brought to 72 millitorr, and the temperature increased to 450.degree. C., when the vacuum is shut off. The increase in pressure in the chamber is then monitored. A leakage rate of no more than 0.5 millitorr per minute is the present standard.
Such CVD chambers are used to deposit metals, such as tungsten, from WF.sub.6 precursor gas. WF.sub.6 is a highly volatile gas, and problems have arisen because tungsten deposits not only on the topside of the wafer, but also on the edge surfaces and backside of the wafer. These edge and backside surfaces are rougher than the highly polished top surface, and are not coated with an adhesive layer such as sputtered titanium nitride, and thus the deposited materials tend to flake off the edge and bottom surfaces, contaminating the chamber. The excess deposits can be etched off in an etch plasma, using the same or a different chamber, but this process itself may form particles in the chamber or damage the backside of the wafers.
Thus clamping rings have come into use. Clamping rings cover the periphery of the wafer during deposition, thereby preventing the deposition gases from reaching the edge and backside surfaces of the wafer. However, due to the volatility of WF.sub.6 for example, clamping rings alone do not prevent edge and backside deposition on the wafer. The use of a purge gas directed behind or at the edge of the wafer behind the clamping ring has also been tried. The purge gas exerts a positive pressure that reduces the chance that processing gas will reach these edge and backside surfaces.
The use of clamping rings has several disadvantages however; the clamping ring is raised and lowered during the processing cycle, and can rub against the susceptor and the wafer, thereby causing particle generation. In addition, clamping rings overlie the peripheral surface of the wafer, reducing the area of the wafer on which metal can be deposited.
Another problem with the use of clamping rings is that the clamping ring, because it is thicker than the wafer, remains cooler than the wafer, and cools the periphery of the wafer where it is in contact with the clamping ring. This causes a drop in deposition rate at the cooler periphery of the wafer, and leads to non-uniformities in the deposited film.
Thus despite the use of all of these features, the deposition of metals such as tungsten by CVD is not as uniform as desired. The use of banks of external high intensity lamps to heat the susceptor and the wafer is not entirely uniform, leading to non-homogeneities in the deposited film. Further, deposits of tungsten and other materials onto the quartz windows builds up over time, reducing the transparency of the windows, so that they must be periodically cleaned. This necessitates opening the chamber and increases downtime, which is expensive. Other problems of particle generation and non-uniform deposition have been noted with the present chambers. Thus the search for the causes of particle generation and non-uniformities in deposited films has continued and solutions to the above problems are being sought continually.